This invention relates to needleless injectors.
In International Patent Publication No. WO 95/03844, a needleless injector is disclosed. This is used to deliver liquid medicaments through a patient""s skin by means of a high pressure jet of the medicament, generated by a piston pump. The energy for operating the pump may be derived from a coiled spring, pyrotechnic charge, hydraulic pressure, or pressurised gas.
Injectors are available using many different types of energy, and the energy may be supplied by the user, for example where a spring is manually compressed and latched to temporarily store the energy until it is required to xe2x80x9cfirexe2x80x9d the injector. Alternatively, the injector may be supplied having the energy already storedxe2x80x94for instance by means of a precompressed spring, or pyrotechnic charge.
Some injectors are intended for disposal after a single use, whereas others have a reoadable energy storage means and a disposable medicament cartridge, and there are many combinations to suit particular applications and markets. For the purposes of the present disclosure, the term xe2x80x9cactuatorxe2x80x9d will be used to describe the energy storage and release mechanism, whether or not it is combined with the medicament cartridge. In all cases, it is necessary to arrange for sufficient force at the end of the piston stroke to deliver all of the medicament at the required pressure: if a spring is used, this is called xe2x80x9cpre-loadingxe2x80x9d.
There is a need to deliver different volumes of medicament according to a patient""s age, weight, and factors relating to the condition being treated, and some single use injectors, such as that disclosed in U.S. Pat. No. 4,913,699 (Parsons), may be capable of delivering different volumes because the user must first fill the injector by using a co-operating medicament transfer device, by which means medicament is transferred from a bulk storage vial into the injection cartridge. Thus the user is able to vary the fill volume and hence the dispensed volume. However these injectors, whilst superficially simple, require the use of the transfer device, which is complicated and difficult to sterilise, and the aseptic transfer of medicament is almost impossible to achieve routinely. Furthermore, to the present inventors"" knowledge, no prior art single use injectors are known which have an actuator mechanism for varying the energy release to suit the amount to be dispensed.
There are more complex re-usable injectors such, as those sold by the MediJect Corporation, in which the actuator may be adjusted to vary the energy release, but these too suffer most of the same fundamental problems as Parson""s injector.
One preferred embodiment in WO 95/03844 is a coil spring powered injector having adjustments for the delivered volume, but even this improvement over the prior art is somewhat complicated to manufacture and operate. Other embodiments in that application describe gas spring powered actuators. (A gas spring comprises a cylinder and piston, with pressurised gas urging the piston which is temporarily restrained by a latch mechanism). A gas spring has several advantages over a coiled spring: in particular, a much higher energy storage density is possible, and at the end of the piston stroke, residual gas may be easily released through an opening in the cylinder wall. This latter point is important to ensure the safe disposal of the injector after use. In the case of a coiled spring-powered actuator, it would require a complicated mechanism to automatically unload the residual energy stored in the spring at the end of its stroke.
Whilst needleless injectors have great potential in reducing pain and fear of needles, and in reducing cross-infection, their high cost compared with a conventional syringe has mitigated against their widespread use, and this is particularly so in relation to injectors which are required to deliver a variable dose.
According to the present invention there is provided an actuator for a gas-powered needleless injector, comprising a drive member movable through a stroke to cause a substance to be delivered by the injector; means for creating a gas pressure on the drive member to cause it to move through the stroke; means for venting the gas pressure after the drive member has travelled through its stroke; and means for adjusting the effective location of the venting means, thereby to adjust the stroke.
A particularly preferred embodiment is an actuator employing a gas spring, as described above, but having a stroke which is easily adjustable in small increments. This is achieved by having a series of valves along the cylinder, so that by opening a particular valve, the pressurised gas driving the piston escapes rapidly through the valve, causing the piston to stop. Although the piston will have some momenturm, in a damped system, which a needleless injector is, the damping and friction will reduce any overtravel to insignificant proportions.
In a first embodiment, a cylinder closed at one end contains a piston which seals and slides against the internal surface of the cylinder. Pressurised gas is contained between the closed end of the cylinder and the piston and urges the piston towards the open end of the cylinder. The piston is temporarily restrained from moving by a latch, operable by a cooperating sliding trigger sleeve. A series of holes aligned longitudinally in the wall of the cylinder between the sealing part of the piston and the open end of the cylinder are covered and sealed by a sleeve slidable over the outside of the cylinder. By moving the sleeve longitudinally the holes may be progressively uncovered, so that when the latch is operated to release the piston, the piston travels in the cylinder bore towards the open end, until the sealing part of the piston passes an uncovered hole, whereupon the pressurised gas escapes rapidly through the hole, and the piston quickly stops. The sliding sleeve has a ratchet or detent to enable the user to preset the sleeve accurately prior to firing the actuator.
In a second embodiment, there is provided an actuator comprising a piston and cylinder as previously described, but having the sliding sleeve movable helically along the outside of the cylinder. In this embodiment the holes through the cylinder wall may also be placed on a helical centreline, so that smaller increments of adjustment are possible.
In yet other embodiments, the piston is connected to a venting member, preferably a rod, which has a vent formed therein, the position of the rod with respect to the piston being variable to adjust the stroke of the piston.
In all embodiments it is desirable that the position of the adjusting means is identified to enable the user to preset the dose.
The effect of shortening the stroke of the piston is to reduce the volume of medicament dispensed from the medicament cartridge, because the piston within the medicament cartridge will also have its stroke reduced. Thus there will be some medicament remaining in the cartridge at the end of the injection. For most medicaments this is unimportant, and in any case, pharmaceutical vials usually contain an excess of the drug required.
Although a gas spring is preferred it is alternatively possible for the gas pressure to be created by a container of gas or vaporisable liquid arranged to apply gas pressure to the drive member upon operation of the actuator.